1. Technical Field
The present invention relates to a printer device which converts image data to print data, and prints the print data onto print media, and also relates to an associated print image-processing device. The invention is suitable for use for an inkjet printer which ejects fine particles (i.e., dots) of ink with different colors onto print media (a recording medium) to thereby form characters and images.
2. Related Art
Along with the proliferation of personal computers and digital cameras, inkjet printers are now widely used not only in offices but also by personal users, since they are generally capable of performing high quality color printing at low cost.
An inkjet printer of one type forms desired characters or images on print media by causing an object, commonly a carriage, which has integrated therein an ink cartridge and a print head, to reciprocate transversely relative to a paper feeding direction, while ejecting (i.e. spraying) particles of liquid ink in dots from nozzles provided on the print head. In addition to monochrome printing, full-color printing can also be readily performed if a printer carriage is provided with ink cartridges for the colors Yellow, Magenta, and Cyan, in addition to Black, with print heads being provided for each of the respective colors. Ink cartridges having six, seven, or even eight ink colors including, for example, Light Cyan and Light Magenta, and so on, in addition to the above four colors are now also commercially available.
The type of inkjet printer described above, however, suffers from a drawback in that a time taken for it to complete a print job is considerably longer as compared to that taken by some other types of printers, such as a laser printer employing electrophotography, which is used in a photocopier. The slow print time of the described carriage-type inkjet printer is due to a large number of reciprocal movements of a print head of the carriage of the printer in a direction transverse to a paper feeding direction of the printer that are required to be made to achieve fine printing. Thus, for fine printing of a single sheet of a print medium, several tens to possibly more than one hundred reciprocal passes in a direction transverse to a paper feeding direction may be required.
There is also known in the art another type of inkjet printer, which does not have a carriage and rather is provided with a print head having a long length capable of traversing a medium to be printed on. In this type of inkjet printer, since there is no need to move a print head in a width-wise direction of a print medium, printing of a print image requires only a single pass of the print head; and therefore printing can be performed at a high speed, similar to that of a laser printer. The former described inkjet printer having a carriage is generally referred to as “a multi-pass printer”; and the latter type is generally referred to as “a line head printer.” In a multi-pass printer, a nozzle-mounted nozzle head is caused to move or scan, for example, in a width-wise direction of a print medium and the print medium itself is also caused to move or scan, for example, in its longitudinal direction. In a line head printer, the print medium only is caused to move or scan in, for example, its longitudinal direction.
In each type of inkjet printer described above, a print head is an indispensable element, and is configured to have a series of microscopic nozzles disposed at set intervals in multiple rows, with each of the nozzles having a diameter of generally between 10-70 micrometers. However, a direction of ejection of ink from the nozzles may be subject to some variation, or skew, due to tilting or misalignment of nozzles arising during the manufacturing process. As a result of such skew, dots of ink ejected from the nozzles may deviate from a target position, giving rise to so-called “splash bending”.
As a result of splash bending and deviation in proper distance between nozzles, printing in an area corresponding to that of such defective nozzles may be subject to so-called “banding”, which can result in considerable degradation in print quality. Such “banding” may manifest on a print medium as either light or dark banding; with light banding appearing when ink dots are formed on a light print medium and distances between neighboring dots are excessively long; and dark banding appearing when such distances are excessively short.
Banding, when it occurs, is far more pronounced when occurring with use of a line head printer as compared to a multi-pass printer. The reason for this is that a line head printer has a print head provided in a fixed position for effecting single pass printing, and also has a considerably larger number of nozzles than are provided in a print head (s) of a multi-pass printer. It is to be noted here, however, that in using a multi-pass printer it is possible to make light banding less noticeable by taking advantage of repeated reciprocal print head movement.
To reduce banding in printing, a software approach referred to as “print control” has been suggested in addition to the hardware approach outlined above.
For example, in the techniques proposed in JP-A-2001-113805, a gradient of each pixel of image data is compared to a threshold, corresponding to each pixel of the image data matrix. In controlling a setting of an ink dot for each pixel, thresholds within the matrix are set depending on print information which is obtained as a result of carrying out a reference print operation using a nozzle head, i.e., a nozzle output characteristic. More specifically, where a line is a column orthogonal to a scanning direction of a print medium, 1) an average resolution is obtained for each respective line of actual print image data; 2) when a dot is formed in a particular cell of a matrix, potentials for other cells of the matrix are obtained based on the average resolution for each line; and 3) when a dot is formed in a cell of the matrix having the smallest potential, potentials for other cells of the matrix are then obtained. These processes are repeated in a set order until dots are formed in each respective cell of the matrix. In the techniques proposed in JP-A-2004-58282, JP-A-2004-58283, and JP-A-2004-58284, a matrix is generated which corresponds to respective pixels of image data, and an effect on a neighboring matrix resulting from formation of an ink dot in a matrix corresponding to a particular pixel is obtained from data of a reference print operation. Consequently, an ink output instruction for each nozzle of a nozzle head can be corrected based on a determination of such an effect.
However, it is difficult to put into practical use the print image processing techniques proposed in the above patent application documents since the above print image processing techniques require a large number of computations. That is, a large number of programs needs to be written; a large number of computations and a large storage capacity are also required. The proposed techniques are also disadvantageous in terms of cost and energy consumption.